This invention relates to remediation of environmental contamination. More particularly, the invention relates to methods for accelerating or enhancing in situ dehalogenation of nonaqueous halogenated solvents in ground water. These methods involve adding to the contaminated ground water a composition of matter that both functions as an electron donor for halorespiration processes carried out by indigenous or exogenously supplied bacteria, wherein the nonaqueous halogenated solvents are dehalogenated and degraded to innocuous compounds, and promotes mass transfer of the nonaqueous halogenated solvents from a source into the ground water where such solvents can be broken down.
For many years little care was taken in the handling of organic solvents and other materials that were used in industry and at government installations, such as military bases. Because of poor handling techniques and, occasionally, intentional dumping, many industrial sites and military bases now have contaminated areas containing relatively high concentrations of these contaminants. Chlorinated solvents, such as trichloroethylene (TCE), perchloroethylene (PCE), and other types of liquids, are common at such sites, and if not removed can infiltrate groundwater supplies, rendering the water unfit for consumption and other uses.
A variety of techniques have been used to promote the removal of such chemical contaminants, both from the soil and from the ground water. The principle method of ground water remediation currently used where dense, non-aqueous phase liquids (DNAPLs) are involved is what is commonly referred to as “pump-and-treat” remediation. According to this method, wells are drilled into the contaminated area and contaminated ground water is pumped above the surface, where it is treated to remove the contaminants.
The limitations of the pump-and-treat method have been documented in articles such as D. M. Mackay & J. A. Cherry, Groundwater Contamination: Pump and Treat Remediation, 23 Environ. Sci. Technol. 630-636 (1989). The authors of this article concluded that pump-and-treat remediation can only be relied on to contain ground water contamination through the manipulation of hydraulic gradients within an aquifer. The reasons for the failure of the pump-and-treat method to decontaminate aquifers are rooted in the limited aqueous solubility of many DNAPLs in ground water and other processes involving contaminant desorption and diffusion. Because of the low aqueous solubility of most DNAPLs, their removal by ground water extraction requires exceptionally long periods of time.
Due to the general impracticability of the pump-and-treat method, considerable attention has been paid recently to other methods for effecting remediation. One such process is commonly referred to as enhanced solubilization. This method uses micellar surfactants to increase the effective solubility of the DNAPLs to accelerate the rate of removal. The mechanism of solubilization displayed by surfactants arises from the formation of microemulsions by the surfactants, water, and the solubilized DNAPLs. For example, Table 1 shows solubilization of PCE by various nonionic and anionic surfactants. These data indicate that even dilute surfactants can significantly increase the aqueous solubility of PCE.
TABLE 1SurfactantPCESurfactantConcentrationSolubilized (mg/l)Water0%240Nonylphenol ethoxylate and2%11,700its phosphate ester (1:1)Sodium diamyl and dioctyl4%85,000sulfosuccinates (1:1) in 500 mgCaCl2/lNonylphenol ethoxylate1%1,300
A serious drawback with the surfactant-enhanced aquifer remediation is that the vertical mobility of the solubilized DNAPLs substantially requires that an aquiclude be present to catch any solubilized contaminant that migrates vertically. Many aquifers, however, lack such an aquiclude. If the traditional surfactant-enhanced aquifer remediation method were to be used with an aquifer lacking an aquiclude, there is a significant risk that the solubilized DNAPLs will spread vertically and contaminate an increasingly large volume. Another drawback of surfactant-enhanced aquifer remediation is the need to pump high concentrations of contaminated water above ground, which results in exposure risks to workers and the environment, and the need to dispose or recycle the surfactant.
Another method for effecting remediation of ground water contaminated with DNAPLs is known as enhanced bioremediation. Enhanced bioremediation, as opposed to intrinsic bioremediation, of halogenated solvent-contaminated ground water falls into the two broad categories of aerobic and anaerobic bioremediation. The aerobic processes, regardless of whether they are carried out in situ or in a bioreactor, require addition of (1) oxygen as the electron acceptor for catabolism of the halogenated solvents, and (2) a carbon source, such as methane, propane, phenol, toluene, or butane. The utilization of an appropriate carbon source induces an enzyme that fortuitously degrades many halogenated solvents, but without any immediate benefit to the microorganisms involved. This process has been applied in situ to aqueous contamination in several instances, and at least one patent has been granted for this approach (U.S. Pat. No. 5,384,048). It has also been used to treat aqueous contamination in above-ground bioreactors with numerous variations, especially using proprietary microorganisms and nutrient mixes. Many patents have been granted in this area, e.g., U.S. Pat. Nos. 5,057,221; 5,962,305; 5,945,331.
Anaerobic bioremediation of halogenated solvents is a fundamentally different process than aerobic bioremediation. Under appropriate anaerobic conditions, chlorinated solvents can be used directly by some microorganisms as electron acceptors through a process that has come to be known as “chlororespiration,” or, more generally, “halorespiration.” D. L. Freedman & J. M. Gossett, Biological Reductive Dechlorination of Tetrachloroethylene and Trichloroethylene to Ethylene Under Methanogenic Conditions, 55 Applied Environ. Microbiol. 2144-2155 (1989), first published the complete degradation pathway for chlorinated ethenes to ethene. In the following years, several publications reported evidence that the degradation could be achieved through microbial respiration, indicating that the microorganisms could actually grow by using chlorinated solvents directly as electron acceptors. The primary requirement to facilitate this process is the addition of a suitable electron donor or carbon source. Many electron donors have been described in the literature, including acetate, lactate, propionate, butyrate, formate, ethanol, hydrogen, and many others. U.S. Pat. No. 5,277,815 issued in 1994 for in situ electron donor addition along with control of redox conditions to effect the desired end products. U.S. Pat. No. 5,578,210 issued later for enhanced anaerobic in situ bioremediation using “biotransformation enhancing agents,” i.e., electron donors such as propylene glycol, glycerol, glutamate, a mixture of proteose peptone, beef extract, yeast extract, malt extract, dextrose, and ascorbic acid, and mixtures thereof. Based primarily on what was publicly available in the scientific literature, studies of enhanced anaerobic in situ bioremediation of chlorinated solvents began in the mid-1990s. This approach generally includes electron donor addition, sometimes with other micronutrients, to facilitate biotransformation of aqueous-phase contaminants. To date, only a few large-scale studies have been published in the peer-reviewed literature, but environmental consulting companies and remediation contractors are increasingly using the general approach.
With one very recent exception, discussed below, all of the work done in this area to date has focused on the biodegradation of aqueous contaminants, because microorganisms cannot directly degrade nonaqueous contaminants. Consequently, bioremediation is not generally thought to be applicable to sites with residual DNAPLs in the subsurface. Therefore, the technologies currently in use include thermal technologies such as steam stripping, in situ chemical oxidation, surfactant flushing, or co-solvent flushing. Surfactant (or co-solvent) flushing, briefly described above, is a chemical process that aims to facilitate transport of nonaqueous contaminants, but without attention to biodegradation. At many sites, however, the pump-and-treat process continues to be used to hydraulically contain residual source areas although it is almost universally accepted that these systems will have to operate in perpetuity because of their inefficient removal of nonaqueous contaminants.
The notable recent exception to the focus of bioremediation on aqueous contaminants away from residual source areas is a study by C. S. Carr et al., Effect of Dechlorinating Bacteria on the Longevity and Composition of PCE-Containing Nonaqueous Phase Liquids under Equilibrium Dissolution Conditions, 34 Environ. Sci. Technol. 1088-1094 (2000), demonstrating that anaerobic bioremediation of tetrachloroethene (PCE) enhanced mass transfer from the nonaqueous phase to the aqueous phase and significantly shortened the longevity of the nonaqueous source. The mechanisms identified were (1) enhanced dissolution of PCE resulting from the continuous removal of the compound from the aqueous phase by bacteria, and (2) increased solubility of the intermediate chlorinated ethenes relative to PCE, allowing the total moles of chlorinated ethenes in the aqueous phase to increase due to biotransformation. This study is important because it identifies some of the advantages of enhancing mass transfer from the nonaqueous phase to the aqueous phase.
In view of the foregoing, it will be appreciated that providing methods for accelerating or enhancing in situ bioremediation of halogenated solvents in ground water would be a significant advancement in the art.